1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus (which will be referred to as an MRI apparatus hereinafter) and a magnetic resonance imaging method that perform imaging of a subject based on a magnetic resonance signal generated in the subjected by applying a gradient magnetic field and a radio-frequency pulse to the subject in a static magnetic field, and use a plurality of transmission/reception radio-frequency coils for transmission of the radio-frequency pulse and reception of the magnetic resonance signal.
2. Description of the Related Art
In recent years, development of parallel imaging has enabled imaging with less distortion at a high speed by an MRI apparatus. Further, the number of coils that can be used in the MRI apparatus or the number of reception channels included in the MRI apparatus tends to increase more and more.
On the other hand, as another tendency in recent years, a static magnetic field intensity is increasing. When the static magnetic field intensity increases, a magnetic resonance frequency rises. When the static magnetic field intensity is, e.g., 3 T, the magnetic resonance frequency is 128 MHz. A wavelength of a radio-frequency pulse becomes short due to such a rise in the magnetic resonance frequency, which results in a factor of nonuniformity of a radio-frequency magnetic field distribution in a living body and unevenness in sensitivity of an image. For example, as a transmission radio-frequency coil, a birdcage type coil having a uniform radio-frequency magnetic field distribution has been conventionally used. In this birdcage type coil, a transmission radio-frequency magnetic field becomes non-uniform due to an increase in frequency, and there occurs an inconvenience that image contrast or uniformity becomes insufficient.
To overcome this problem, a method using an array coil as a transmission coil attracts attention (see Katscher U et al: Transmit SENSE. Magn Reson Med 49: 144-150 (2003)). According to this method, radio-frequency magnetic fields having different amplitudes or phases are applied from respective array coils arranged to surround a subject, thereby realizing uniform excitation with respect to the subject.
On the other hand, the array coil has been already used as a reception coil. When using a transmission array coil separately from such a reception array coil, both the transmission array coil and the reception array coil must be disposed in a gantry, around a subject, or to a bed.
Further, when using different array coils for transmission and reception, a large transmission array coil that can cover a subject and a reception array coil is prepared. Moreover, in this case, a large transmission power is required, which is not economical. Furthermore, applying a radio frequency to a wide region of the subject may possibly lead to an increase in a specific absorption rate (SAR) of the subject. That is, applying the radio-frequency magnetic field only to a region that should be imaged is desirable, but this is difficult to be realized in the above-explained structure.
Using the transmission array coil and the reception array coil as explained above results in occurrence of various inconveniences.
Moreover, using the large transmission coil that can cover the large region to enable imaging at various positions in a wide region results in occurrence of various inconveniences.
Thus, enabling one array coil to be used for both transmission and reception is naturally desired, but a magnetic resonance imaging apparatus that realizes this structure is unknown.
Additionally, enabling changing a region to which a radio-frequency magnetic field is applied by the transmission coil is desired, but a magnetic resonance imaging apparatus that realizes this structure is not known.